I. Field of the Invention
The present invention relates to a sensor for producing pulses in response to relative movement between the sensor and a graticule.
II. Description of the Prior Art
Optical incremental sensors are widely used to monitor rotary and linear motion. Optical incremental sensors typically generate a symmetrical repeating waveform that is utilized to monitor relative motion between the sensor and another item. The basic components of such optical sensors typically include a light source, a light shutter (including a graticule and a mask), a light sensor, and an encoder circuit which operates upon the output of the light sensor to provide an output signal.
One form of such prior art optical incremental sensor is illustrated in FIG. 1 as comprising a light source 10, a graticule 12, light detectors 14 and 16, and masks 18 and 20. Graticule 12 typically consists of a plurality of equal width alternate opaque and non-opaque portions 22, 24. Masks 18 and 22 also comprise a plurality of alternating equal width opaque and non-opaque portions 26, 28.
Masks 18 and 20 are aligned between graticule 12 and sensors 14 and 16 such that when light from source 10 passes through graticule 12, mask 18 is aligned so as to permit this light to strike sensor 14 whereas mask 20 at that moment is aligned so as to prevent such light from reaching sensor 16. As graticule moves with respect to masks 18 and 20, eventually light passing through graticule 12 is blocked by mask 18 from reaching sensor 14 whereas at precisely that moment light passing through graticule 12 is permitted to pass through mask 20 and reach sensor 16. Accordingly, an output A of sensor 14 is exactly 180 degrees out of phase with an output B of sensor 16. When the output A is subtracted from output B, the resultant signal is indicative of the location of graticule 12. More specifically, the resultant difference signal between output A and output B, when sensors 14 and 16 are perfectly balanced, will cross an average value each time graticule 12 is moved the width of one of opaque or non-opaque sections 22, 24.
However, it is very difficult to align masks 18 and 20 in the proper orientation to be sure that the output of sensor 14 will be precisely 180 degrees out of phase with the output of sensor 16. Moreover, prior art of the type illustrated in FIG. 1 has the disadvantage of requiring the utilization of new masks each time the pitch of graticule 12 (the total width of an opaque portion 22 and a non-opaque portion 24) is altered, since the pitch of graticule 12 must correspond with the pitch of masks 18 and 20. Accordingly, each time a graticule with a different pitch is employed, new masks must be carefully positioned 180 degrees out of phase with one another.
It is, accordingly, an object of the subject invention to provide a graticule sensor which provides a highly accurate output without the necessity of incorporating masks.
It is also an object of the present invention to provide a graticule sensor in which the same sensor can be utilized when changes in graticule pitch are made.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention.